Reticles for lithography

ABSTRACT

An example method for making a reticle includes providing an assembly. The assembly includes an extreme ultraviolet mirror and a cavity overlaying at least a bottom part of the extreme ultraviolet mirror. The method also includes at least partially filling the cavity with an extreme ultraviolet absorbing structure that includes a metallic material that includes an element selected from Ni, Co, Sb, Ag, In, and Sn, by forming the extreme ultraviolet absorbing structure selectively in the cavity.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a non-provisional patent application claimingpriority to European Patent Application No. 17190264.6, filed Sep. 9,2017, the contents of which are hereby incorporated by reference.

FIELD OF THE DISCLOSURE

The present disclosure is related to the field of lithography. Morespecifically, it is related to fabrication of reticles potentiallysuitable for Extreme Ultra-Violet Lithography (EUVL).

BACKGROUND

The desire to reduce feature sizes in IC manufacturing, based on Moore'sLaw, incentivizes the transition to EUVL for patterning features havinga critical dimension (CD) of less than 10 nm. EUVL generally involvesthe use of light having a wavelength of 13.5 nm to enable singleexposure patterning of features having a dimension smaller than 10 nm,potentially making it possible to keep up with Moore's Law.

The absorber material present above the multi-layer mirror in thereticles used for EUVL typically plays an important role in terms ofabsorption of EUV light during lithography. Therefore, it is desirablethat the absorber material possesses a high extinction coefficient ork-value. Currently, Ta-based absorber materials are used in EUV reticleshaving a typical thickness of 70 nm. However, these traditional EUVmasks suffer from Mask 3D (M3D) effects such as shadowing differencesbetween horizontal and vertical patterns, best focus shifts throughpitch, and pattern shifts through focus. M3D effects result from theinteraction between the oblique incident EUV light and the patternedabsorber material with typical thickness values on the order of severalwavelengths. M3D effects can make it difficult to perform opticalproximity correction, and can lead to overlay problems. It has beenshown that reducing the thickness of the current Ta-based absorbermaterials is generally not sufficient to mitigate M3D effects (“ReducingEUV mask 3D effects by alternative metal absorbers”, ConferenceProceedings of SPIE 2017).

Furthermore, in addition to the absorber material having suitablematerial properties making it compatible for use in reticles in an EUVscanner environment and making it environmentally friendly, it shouldalso be compatible with the current EUV reticle fabrication processes.This compatibility generally should ensure obtaining absorber profilesas square as possible, the absence of mask defects on the mask, and theabsence of damage on the capping layer protecting the multi-layer mirroror on the multi-layer mirror itself, during reticle fabrication. Theabsorber material is typically provided by blanket layer depositionmethods or by non-selective deposition methods on the multi-layermirror, which can necessitate further the use of etch-back or chemicalmechanical planarization (CMP) techniques to arrive at the desiredabsorber patterns. However, these techniques can make the manufacturingcost of reticles not only expensive due to the introduction of thesefurther steps, but also etch-back or CMP techniques can damage the Rucapping and/or the multi-layer mirror, which is typically undesirable.

There is, therefore, a need in the art for methods for fabricating EUVreticles, which help resolve some or all of the issues outlined above.

SUMMARY

Embodiments of this disclosure can help form EUV reticles and provideEUV reticles or intermediates in the provision thereof.

Embodiments of this disclosure can help attain EUV absorbing structureshaving sidewalls that are straight, i.e., substantially free from anglesand not curved when the surface roughness is neglected.

Embodiments of this disclosure can help attain EUV absorbing structureshaving sidewalls that are vertical or close to vertical.

Embodiments of this disclosure can help form EUV absorbing structureshaving zero no or negligible defects on the mask, which means thatduring patterning no or a few damages are created on the EUV mirror orits capping layer and therefore its reflective properties are notaltered where no EUV absorbing structure is intended. In the comparativesubtractive method, EUV absorbing materials removed by physical etching,such as Ni or Co, tend to deposit on the EUV mirror outside of where theEUV absorbing material is desired.

Embodiments of this disclosure can help attain EUV absorbing structureshaving no voids therein.

Embodiments of this disclosure can help attain EUV absorbing structuresthat include amorphous or polycrystalline material having an averagecrystallite size of 10 nm or smaller. This translates into an EUVabsorbing structure having relatively smooth side walls.

Embodiments of this disclosure can help attain a capping layer having noor few damages. Within embodiments of this disclosure, the capping layeris typically left undamaged in order to keep its protective propertiesfor the EUV mirror underneath.

Embodiments of this disclosure can help attain EUV absorbing structuresabove the EUV mirror having low or no internal stress so that no orlittle bending of the reticle patterns occurs.

Embodiments of this disclosure can help attain EUV absorbing structuresthat are stable in the temperature range 20-250° C., e.g., 20-180° C.

Embodiments of this disclosure can help attain a width as small as oreven smaller than 64 nm for the EUV absorbing structure, whichtranslates into features as small as or even smaller than 16 nm on thewafer at a numerical aperture (NA) of 0.33. Embodiments of the presentdisclosure can even permit the use of a width much smaller than 64 nmfor the EUV absorbing structure, such as a width of 24 nm, whichtranslates into features as small as 6 nm on the wafer a numericalaperture (NA) of 0.33.

Embodiments of this disclosure can help attain reduced manufacturingcost of EUV reticles when compared to subtractive or non-selectivemethods.

The above objective is accomplished by a method and device according tothe present disclosure.

In a first aspect, the present disclosure relates to a method for makinga reticle, comprising: providing an assembly comprising: an extremeultraviolet mirror; and a cavity overlaying at least a bottom part ofthe extreme ultraviolet mirror; and at least partly filling the cavitywith an extreme ultraviolet absorbing structure comprising a metallicmaterial comprising an element selected from Ni, Co, Sb, Ag, In, and Sn,by forming the extreme ultraviolet absorbing structure selectively inthe cavity.

In some embodiments, the present disclosure relates to a method formaking an extreme ultraviolet reticle, comprising: providing an assemblycomprising: an extreme ultraviolet mirror; and a cavity overlaying atleast a bottom part of the extreme ultraviolet mirror, the cavity beingeither: comprised in a dielectric mask layer overlaying the extremeultraviolet mirror, the cavity having a depth extending from a topsurface of the dielectric mask layer to a bottom surface of thedielectric mask layer, or comprised in the extreme ultraviolet mirror,the cavity having a depth extending from a top surface of the extremeultraviolet mirror to a level above the bottom surface of the extremeultraviolet mirror, and filling the cavity with an extreme ultravioletabsorbing structure comprising a metallic material by forming theextreme ultraviolet absorbing structure selectively in the cavity.

In another embodiment, the present disclosure relates to a method formaking an ultraviolet reticle, comprising: providing an assemblycomprising: an extreme ultraviolet mirror; and a cavity overlaying atleast a bottom part of the extreme ultraviolet mirror; at least partlyfilling the cavity with an extreme ultraviolet absorbing structurecomprising a metallic material comprising an element selected from Ni,Co, Sb, Ag, In, and Sn, by forming the extreme ultraviolet absorbingstructure selectively in the cavity.

In another embodiment, the present disclosure relates to an extremeultraviolet reticle, comprising: an extreme ultraviolet mirror; and anextreme ultraviolet absorbing structure comprising a metallic materialcomprising an element selected from Ni, Co, Sb, Ag, In, and Sn.

In another embodiment, the present disclosure relates to an intermediatestructure for the making of an extreme ultraviolet reticle, theintermediate structure comprising an extreme ultraviolet mirror; a masklayer over the extreme ultraviolet mirror, the mask layer beingpatterned with a cavity extending from a top surface to a bottom surfaceof the mask layer; and an extreme ultraviolet absorbing structurefilling at least partly the cavity, the extreme ultraviolet absorbingstructure comprising a metallic material comprising an element selectedfrom Ni, Co, Sb, Ag, In, and Sn.

Various aspects of the disclosure are set out in the accompanyingindependent and dependent claims. Features from the dependent claims maybe combined with features of the independent claims and with features ofother dependent claims as appropriate and not merely as explicitly setout in the claims.

The above and other characteristics, features and advantages of thepresent disclosure will become apparent from the following detaileddescription, taken in conjunction with the accompanying drawings, whichillustrate, by way of example, the principles of the disclosure. Thisdescription is given for the sake of example only, without limiting thescope of the claims. The reference figures quoted below refer to theattached drawings.

BRIEF DESCRIPTION OF THE FIGURES

The above, as well as additional, features will be better understoodthrough the following illustrative and non-limiting detailed descriptionof example embodiments, with reference to the appended drawings.

FIG. 1 is a schematic representation of a vertical cross-section ofintermediates in the formation of a reticle during a method according toan embodiment of the present disclosure.

FIG. 2 is a schematic representation of a vertical cross-section ofintermediates in the formation of a reticle during a method according toan embodiment of the present disclosure.

FIG. 3 is a schematic representation of a vertical cross-section ofintermediates in the formation of a reticle during a method according toan embodiment of the present disclosure.

FIG. 4 is a schematic representation of a vertical cross-section ofintermediates in the formation of a reticle during a method according toan embodiment of the present disclosure.

FIG. 5 is a schematic representation of a vertical cross-section ofintermediates in the formation of a reticle during a method according toan embodiment of the present disclosure.

FIG. 6 is a schematic representation of a vertical cross-section ofintermediates in the formation of a reticle during a method according toan embodiment of the present disclosure.

FIG. 7 is a schematic representation of a vertical cross-section ofintermediates in the formation of a reticle during a method according toan embodiment of the present disclosure.

FIG. 8 is a schematic representation of a vertical cross-section ofintermediates in the formation of a reticle during a method according toan embodiment of the present disclosure.

FIG. 9 is a schematic representation of a vertical cross-section ofintermediates in the formation of a reticle during a method according toan embodiment of the present disclosure.

FIG. 10 is a schematic representation of a vertical cross-section ofintermediates in the formation of a reticle during a method according toan embodiment of the present disclosure.

FIG. 11 is a schematic representation of a vertical cross-section ofintermediates in the formation of a reticle during a method according toan embodiment of the present disclosure.

FIG. 12 is a schematic representation of a vertical cross-section ofintermediates in the formation of a reticle during a method according toan embodiment of the present disclosure.

FIG. 13 is a schematic representation of a vertical cross-section ofintermediates in the formation of a reticle during a method according toan embodiment of the present disclosure.

FIG. 14 is a schematic representation of a vertical cross-section ofintermediates in the formation of a reticle during a method according toan embodiment of the present disclosure.

FIG. 15 is a schematic representation of a vertical cross-section ofintermediates in the formation of a reticle during a method according toan embodiment of the present disclosure.

FIG. 16 is a schematic representation of a vertical cross-section ofintermediates in the formation of a reticle during a method according toan embodiment of the present disclosure.

FIG. 17 is a schematic representation of a vertical cross-section ofintermediates in the formation of a reticle during a method according toan embodiment of the present disclosure.

FIG. 18 is a schematic representation of a vertical cross-section ofintermediates in the formation of a reticle during a method according toan embodiment of the present disclosure.

FIG. 19 is a schematic representation of a vertical cross-section ofintermediates in the formation of a reticle during a method according toan embodiment of the present disclosure.

FIG. 20 is a schematic representation of a vertical cross-section ofintermediates in the formation of a reticle during a method according toan embodiment of the present disclosure.

FIG. 21 is an electron micrograph showing a vertical cross-section of anassembly without an EUV mirror, according to an embodiment of thepresent disclosure.

FIG. 22 is an electron micrograph showing a vertical cross-section ofanother assembly without an EUV mirror, according to an embodiment thepresent disclosure.

FIG. 23 is an electron micrograph showing a vertical cross-section ofyet another assembly without an EUV mirror, according to an embodimentthe present disclosure.

In the different figures, the same reference signs generally refer tothe same or analogous elements.

All the figures are schematic, not necessarily to scale, and generallyonly show parts which are necessary to elucidate example embodiments,wherein other parts may be omitted or merely suggested.

DETAILED DESCRIPTION

Example embodiments will now be described more fully hereinafter withreference to the accompanying drawings. That which is encompassed by theclaims may, however, be embodied in many different forms and should notbe construed as limited to the embodiments set forth herein; rather,these embodiments are provided by way of example. Furthermore, likenumbers refer to the same or similar elements or components throughout.

Embodiments of the disclosure will be described with reference tocertain drawings but the scope of the claims is not limited thereto. Thedrawings described are only schematic and are non-limiting. In thedrawings, the size of some of the elements may be exaggerated and notdrawn to scale for illustrative purposes.

Furthermore, the terms first, second, third, and the like in thedescription and in the claims, are used for distinguishing betweensimilar elements and not necessarily for describing a sequence, eithertemporally, spatially, in ranking, or in any other manner. It is to beunderstood that the terms so used are interchangeable under appropriatecircumstances and that the embodiments of the disclosure are capable ofoperation in other sequences than described or illustrated herein.

Moreover, the terms top, bottom, over, under, and the like in thedescription and the claims are used for descriptive purposes and notnecessarily for describing relative positions. It is to be understoodthat the terms so used are interchangeable under appropriatecircumstances and that the embodiments of the disclosure are capable ofoperation in other orientations than described or illustrated herein.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present disclosure. Thus, appearancesof the phrases “in one embodiment” or “in an embodiment” in variousplaces throughout this specification are not necessarily all referringto the same embodiment, but may. Furthermore, the particular features,structures, or characteristics may be combined in any suitable manner,as would be apparent to one of ordinary skill in the art from thisdisclosure, in one or more embodiments.

Similarly, it should be appreciated that in the description ofembodiments of the disclosure, various features of the embodiments aresometimes grouped together in a single embodiment, figure, ordescription thereof for the purpose of streamlining the disclosure andaiding in the understanding of one or more of the various inventiveaspects. This method of disclosure, however, is not to be interpreted asreflecting an intention that any claim requires more features than areexpressly recited in each claim. Rather, as the following claimsreflect, inventive aspects lie in less than all features of a singleforegoing disclosed embodiment. Thus, the claims following the detaileddescription are hereby expressly incorporated into this detaileddescription, with each claim standing on its own as a separateembodiment of this disclosure.

Furthermore, while some embodiments described herein include some butnot other features included in other embodiments, combinations offeatures of different embodiments are meant to be within the scope ofthe disclosure, and form different embodiments, as would be understoodby those in the art. For example, in the following claims, any of theclaimed embodiments can be used in any combination.

In the description provided herein, numerous specific details are setforth. However, it is understood that embodiments of the disclosure maybe practiced without these specific details. In other instances,well-known methods, structures, and techniques have not been shown indetail in order not to obscure an understanding of this description.

As used herein, and unless otherwise specified, the term extremeultraviolet (EUV) may be understood to refer to a part of theelectromagnetic spectrum spanning wavelengths from 124 nm down to 10 nm.

As used herein, and unless otherwise specified, the refractive index (n)and extinction coefficient (κ) are respectively the real and imaginaryparts of the complex index of refraction ({right arrow over (n)}=n−iκ).Their values herein can refer to measurements performed at a wavelengthof 13.5 nm. 13.5 nm is a commonly used wavelength for extremeultraviolet lithography, though the disclosure is in no way limited tothis specific wavelength. Both the refractive index and the extinctioncoefficient can, for example, be obtained from angle resolvedreflectance measurements; or from angle-and-wavelength resolvedreflectance measurements (thereby increasing the redundancy available tofit both n an κ). Other possibilities available to the skilled personinclude, but are not limited to, transmittance and/or absorbancemeasurements.

As used herein, the average crystallite size of an alloy can refer tothe average size of crystals within the alloy. It may also be referredto as the grain size.

Embodiments of the disclosure will now be described. It is clear thatother embodiments of the disclosure can be configured according to theknowledge of persons skilled in the art without departing from thetechnical teaching of the disclosure.

In some embodiments, the present disclosure relates to a method formaking an extreme ultraviolet reticle. The method comprises providing anassembly comprising: (i) an extreme ultraviolet mirror; and (ii) acavity overlaying at least a bottom part of the extreme ultravioletmirror.

The extreme ultraviolet mirror will typically be on a substrate. Thesubstrate may be any substrate but is typically a substrate suitable foruse in manufacturing a mask for lithography. The substrate, may forexample, be a quartz substrate. The extreme ultraviolet mirror may beany mirror suitable for reflecting extreme ultraviolet radiations. TheEUV mirror may be a Bragg reflector comprising a multilayer stack of afirst material alternating with a second material, wherein the first andsecond material have different refractive indices and wherein thethickness of the layers is such that the optical path length within eachlayer corresponds to a quarter of the wavelength of the incident light(e.g. 13.5 nm EUV light). For instance, it can be a stack of Si layersalternated with Mo layers. In typical embodiments, a capping layer willbe present on the extreme ultraviolet mirror. The capping layer istypically a Ru layer but can be any layer or multilayer suitable forprotecting the extreme ultraviolet mirror from environmentaldegradation. Examples of such a layer or multilayer are a Rh layer or aTiO₂/RuO₂ multilayer. The thickness of the capping layer is generallychosen so that it does not reduce the reflectance at a 13.5 nmwavelength by more than 3%. The thickness of the capping layer can befor instance from 2 to 3 nm, e.g., 2.5 nm.

The assembly further comprises a cavity overlying at least a bottom partof the extreme ultraviolet mirror.

In some embodiments, the cavity may be overlying part of the top surfaceof the EUV mirror. In these embodiments, the cavity may expose part ofthe capping layer typically present on the EUV mirror or may expose partof the etch stop layer when such a layer is present above the EUV mirrorand typically on the capping layer. The cavity may alternatively exposepart of a seeding layer if such a layer is present on the capping layer.The seeding layer is generally a layer which promotes the selectiveformation of the EUV absorbing structure in the cavity. Typically, theseeding layer is a layer having the same chemical nature as the EUVabsorbing structure. In these embodiments, the cavity is typicallyformed by providing a dielectric mask layer over the EUV mirror followedby patterning the dielectric mask layer so as to form the cavity.

The cavity may be provided by providing a dielectric patterned templatehaving a cavity, or dielectric mask features separated by a cavity,having a depth extending from an upper surface of the patterned templatethroughout its thickness.

In embodiments, the mask layer may be formed of SiO₂. The SiO₂ masklayer is generally formed at a temperature below 250° C. and moretypically below 180° C. Patterning the dielectric mask layer can beperformed for instance by providing a spin-on-carbon (SoC) layer on thedielectric mask layer, providing a spin-on-glass layer (SoG) on the SoClayer, and forming a patterned photoresist on the SoG layer. Thepatterned photoresist can present an opening suitable for forming thecavity in the mask layer upon transfer therein. This transfer istypically obtained by etching through the SoG layer, the SoC layer, andthe mask layer, and stopping at the capping layer.

In embodiments, the dielectric mask layer may be formed of SoC.Patterning the SoC mask layer can be performed, for instance, byproviding a SoG layer on the SoC layer, and forming a patternedphotoresist on the SoG layer. The patterned photoresist can present anopening suitable for forming the cavity in the mask layer upon transfertherein. This transfer is typically obtained by etching through the SoGlayer and the SoC mask layer and stopping at the capping layer or on theetch stop layer if present.

In some embodiments, the cavity may be formed by providing a photoresistlayer directly on the capping layer of the EUV mirror followed bypatterning the photoresist layer so as to form the cavity in thephotoresist. It is this cavity that can be at least partly filled withan extreme ultraviolet absorbing structure.

In some embodiments, the cavity may extend in the extreme ultravioletmirror and the bottom of the cavity may be lower than the top surface ofthe extreme ultraviolet mirror but higher than the bottom surface of theextreme ultraviolet mirror. To form such a cavity, an extremeultraviolet mirror may be provided on a substrate, a capping layer maybe provided on the extreme ultraviolet mirror, a dielectric layer (e.g.SiO₂ or Si₃N₄) may be provided on the capping layer, an SoC layer may beprovided on the dielectric layer, an SoG layer may be provided on theSoC layer, and a patterned photoresist may be provided on the SoG layer.The patterned photoresist can present an opening suitable for formingthe cavity in the EUV mirror layer upon transfer therein. This transferis typically obtained by etching through the SoG layer, the SoC layer,the dielectric layer, the capping layer, and the EUV mirror layer andstopping before reaching the bottom of the EUV mirror layer. The exactdepth at which the etching stops may be controlled, for instance, byplacing an etch stop layer at that depth in the EUV mirror.

In various embodiments, the cavity is overlaying at least a bottom partof the EUV mirror. In some embodiments, the cavity is situated above thelevel of the top surface of the EUV mirror and typically the lateralextent of the cavity overlaps with part of the lateral extent of the topsurface of the EUV mirror. In some embodiments, the bottom of the cavityis situated above the level of the bottom surface of the EUV mirror butbelow the level of the top surface of the EUV mirror. Typically, thelateral extent of the cavity overlaps with part of the lateral extent ofthe bottom surface of the EUV mirror.

The cavity is typically defined by side walls and by a bottom surface.In embodiments, the side walls are formed of a dielectric material whilein other embodiments, the sidewalls are formed of: an EUV mirrormaterial, typically a capping material on the EUV mirror material,typically a self-assembled monolayer (SAM) material on the cappingmaterial.

The surfaces forming the cavity include the bottom of the cavity and itsside walls. Each side wall generally forms an angle with the top surfaceon the EUV mirror ranging from 70° to 110° or from 75° to 105°.

The width of the cavity may, for instance, be from 3 nm to 300 nm orfrom 8 nm to 100 nm.

The material of the mask layer is generally non-crystalline. This is canbe advantageous as it may contribute to the sidewalls of the cavityhaving a lower roughness. (This can be helpful in that when absorbermaterial is selectively deposited and later when the mask is removed,the absorber patterns do not suffer much from Line Edge Roughness andLine Width Roughness.

In embodiments, a method includes at least partly filling the cavitywith an EUV absorbing structure by forming the EUV absorbing structureselectively in the cavity. In other words, the EUV absorbing structurecan be formed selectively in the cavity with respect to any surface notforming part of the cavity.

By at least partly filling the cavity, it is generally meant thatalthough the whole lateral extent of the cavity is filled, the cavitymay either be filled completely or filled up to a certain height belowthe top of the cavity.

In some embodiments, filling the cavity may be performed by electrolessdeposition (ELD) of the filling material.

In embodiments, the extreme ultraviolet absorbing structure may beformed selectively in the cavity by electroless deposition.

As explained in the background, conventional Ta-based absorbermaterials, due to their relatively low extinction coefficient value k,tend to require a relatively large thickness to be sufficientlyabsorbing. Such a large thickness can lead to M3D effects whichgenerally cannot be entirely mitigated by simply reducing thatthickness. The use of alternative absorber materials having a higherextinction coefficient k than current Ta-based materials but possessinga similar refractive index n (e.g., between 0.86 and 1.02, or between0.88 and 1.00 at 13.5 nm) can help reduce the thickness of the EUVabsorbing structure but typically requires a new formation method. Asshown in the comparative example infra, finding suitable methods makinguse of such materials was a challenge. Embodiments of this disclosurehelp address that challenge.

Generally, and especially when the filling material is metallic, ELD maybe used as it can be well suited to the selective deposition of metallicmaterials. The filling material generally forms a layer of electricallyconductive material (e.g., a metallic layer) in the cavity. When the EUVabsorbing structure is a multilayer, the bottom layer is generally aconductive material and more typically a metallic material. Conductivematerials and metallic materials are preferred because they permit thefilling step to be performed selectively by ELD. In particular, they canpermit selective deposition on a Ru cap layer by ELD. Metallic materialsare generally well suited for forming the EUV absorbing structure bymethods of the present disclosure because they tend to have a highextinction coefficient k and they can be difficult to form by othermethods.

In embodiments, the extreme ultraviolet absorbing structure may comprisea metallic material. In some embodiments, the extreme ultravioletabsorbing structure may consist of a metallic material.

The metallic material may for instance be a single metal, an alloy, or adoped metal or alloy.

Suitable metallic materials are typically selected from transitionmetals, post-transition metals, Sb and combination thereof.

Generally, the EUV absorbing structure has an extinction coefficient kof at least 0.040, more typically at least 0.050, even more typically atleast 0.060 and most typically at least 0.065 at a wavelength of 13.5nm. Such a high extinction coefficient k permits the thickness of theEUV absorbing structure to be less than 40 nm or less than 35 nm whileremaining sufficiently absorbing. For instance, the thickness of the EUVabsorbing structure may be from 25 to 35 nm. This represents animprovement when compared to using TaBN for the EUV absorbing structurewhere this thickness generally must be at least 60 nm.

In embodiments, the EUV absorbing structure may comprise a metallicmaterial comprising an element selected from Ni, Co, Sb, In, Sn, and Ag.

In embodiments, the EUV absorbing structure may comprise a layerconsisting of an element selected from Ni, Co, Sb, In, Sn, and Ag.

In embodiments, the EUV absorbing structure may comprise a layerconsisting of an alloy of an element selected from Ni, Co, Sb, In, Sn,and Ag with one or more other elements. Generally, the resulting alloyhas an extinction coefficient k of at least 0.040, more typically atleast 0.050, even more typically at least 0.060 and most typically atleast 0.065 at a wavelength of 13.5 nm. For instance, the EUV absorbingstructure may comprise a layer of NiPt. Generally, at least 50% atomiccomposition of the alloy is made of elements selected from Ni, Co, Sb,In, Sn, and Ag.

In embodiments, the EUV absorbing structure may consist in a singlelayer of an element selected from Ni, Co, Sb, In, Sn, and Ag.

In embodiments, the EUV absorbing structure may consist of a singlelayer consisting of an alloy of an element selected from Ni, Co, Sb, In,Sn, and Ag with one or more other elements. Generally, the resultingalloy has an extinction coefficient k of at least 0.040, more typicallyat least 0.050, even more typically at least 0.060 and most typically atleast 0.065 at a wavelength of 13.5 nm. For instance, the EUV absorbingstructure may comprise a layer of NiPt. Generally, at least 50% atomiccomposition of the alloy is made of elements selected from Ni, Co, Sb,In, Sn, and Ag.

In embodiments, the extreme ultraviolet absorbing structure may compriselayers of extreme ultraviolet absorbing metallic material comprising anelement selected from Ni, Co, Sb, In, Sn, and Ag (e.g. having anextinction coefficient k of at least 0.040, more typically at least0.050, even more typically at least 0.060 and most typically at least0.065 at a wavelength of 13.5 nm) alternated with spacer layers.

In embodiments, the EUV absorbing structure may consist of alternatinglayers of an element selected from Ni, Co, Sb, In, Sn, and Ag withspacer layers.

In embodiments, the EUV absorbing structure may consist of alternatinga) layers of an alloy of an element selected from Ni, Co, Sb, In, Sn,and Ag with one or more other element, with b) spacer layers. Generally,at least 50% atomic composition of the alloy is made of elementsselected from Ni, Co, Sb, In, Sn, and Ag.

In some embodiments, the EUV absorbing structure may comprise one ormore doped metal layers, e.g. metal layers doped with up to 20% atomiccomposition of B or P.

Generally, the resulting alloy involved in the layer described a) has anextinction coefficient k of at least 0.040, more typically at least0.050, even more typically at least 0.060, and most typically at least0.065 at a wavelength of 13.5 nm. For instance, these layers of an alloymay be layers of NiPt.

The elements Ni, Co, Sb, In, Sn and Ag can be advantageous as they havean extinction coefficient k of at least 0.065 at 13.5 nm and they can beselectively grown on the bottom of the cavity by ELD.

Generally, the filling material consists of Ni, Co, Ag, CoWP, Ni(B) orNiPt. Ni(B) is Ni doped with up to 20% atomic composition of B. Mosttypically, the filling material consists of Ni or Co. These twomaterials are metals and can be grown by ELD. Sn is typically avoided inEUV scanners. The melting point of Sn is very low which can also be adisadvantage. Sb is also generally avoided in EUV scanners because it istoxic when forming volatile hydrogen compounds such as H₃Sb. Ag has ahigher tendency to crystallize than Co or Ni, leading to EUV absorbingstructures having rough side walls, which is generally not preferred.

The spacer layers are made of a material different from Ni, Co, Sb, In,Sn, and Ag; and different from an alloy of an element selected from Ni,Co, Sb, In, Sn, and Ag with one or more other element. The spacer layersare typically not metallic. The spacer layers are typically made of anamorphous material. Examples of suitable spacer layers are TiN and MgO.Generally, the spacer layers are selected in such a way that the sidewalls of the cavity can be removed without removing the spacer layers.

Embodiments of this disclosure can be performed at a temperature lowerthan 250° C., typically lower than 180° C., more typically lower than150° C. and yet more typically not larger than 100° C. This can resultin a low thermal budget and hence a low consumption of energy. Theseembodiments also have the potential to prevent damaging the EUV mirror.

In embodiments, the assembly provided may further comprise: aself-assembled monolayer of a first type present in the cavity forpromoting the selective formation of the extreme ultraviolet absorbingstructure therein, and/or a self-assembled monolayer of a second typepresent on a surface not forming part of the cavity for preventing orinhibiting the formation of the extreme ultraviolet absorbing structurethereon.

In embodiments, the assembly may further comprise a capping layer on theextreme ultraviolet mirror and the cavity may expose part of the cappinglayer.

In embodiments, the assembly may further comprise a capping layer on theextreme ultraviolet mirror and an etch stop layer on the capping layer,wherein the cavity exposes part of the etch stop layer.

In embodiments, the cavity may extend into the extreme ultravioletmirror and the extreme ultraviolet absorbing structure may be partlyembedded in the extreme ultraviolet mirror so that its bottom surface islower than the top surface of the extreme ultraviolet mirror and thatits top surface is higher or at the same level as the top surface of theextreme ultraviolet mirror.

In embodiments, a seed layer may overlie the extreme ultraviolet mirrorand the cavity may open on the seed layer.

In embodiments, the cavity may be formed by providing a mask layer overthe extreme ultraviolet mirror followed by patterning the mask layer soas to form the cavity. In these embodiments, the method may furthercomprise removing the mask layer selectively with respect to: (i) theextreme ultraviolet absorbing structure, and (ii) the extremeultraviolet mirror or the capping layer if present.

In embodiments, a thickness of the extreme ultraviolet absorbingstructure may be 60 nm or below, 50 nm or below, or 35 nm or below.

In embodiments, the present disclosure relates to an extreme ultravioletreticle obtainable by any method embodiments of the disclosure.

In embodiments, the present disclosure relates to an extreme ultravioletreticle, comprising: an extreme ultraviolet mirror; and a metallicextreme ultraviolet absorbing structure comprising an element selectedfrom Ni, Co, Sb, Ag, In, and Sn.

In particular, the metallic extreme ultraviolet absorbing structure mayhave straight side walls.

In particular, the metallic extreme ultraviolet absorbing structure maybe formed of Ag, CoWP, Ni doped with B or NiPt; typically Ni or Co.

In particular, the metallic extreme ultraviolet absorbing structure mayhave side walls forming an angle with the top surface on the EUV mirrorof from 70° to 110°, typically from 75° to 105°.

In embodiments, the present disclosure further relates to anintermediate structure for the making of an extreme ultraviolet reticle,the intermediate structure comprising an extreme ultraviolet mirror; amask layer over the extreme ultraviolet mirror, the mask layer beingpatterned with a cavity extending from a top surface to a bottom surfaceof the mask layer; and an extreme ultraviolet absorbing structurefilling at least partly the cavity, the extreme ultraviolet absorbingstructure comprising an a metallic material comprising an elementselected form Ni, Co, Sb, Ag, In, and Sn.

In this aspect, since the extreme ultraviolet absorbing structure isgenerally embedded laterally within a patterned dielectric template,which covers the capping layer typically present on the multi-layermirror, the capping layer and the mirror will generally be protectedagainst any damage that might happen if this intermediate reticlestructure needs to be transported somewhere else or to another processtool.

EXAMPLE 1 Formation of a Reticle Including a Patterning Step into anSiO₂ Core

Example 1 is related to the formation of a reticle using SiO₂ as atemplate for the area selective deposition of Co or Ni (or compoundscomprising Co or Ni). We now refer to FIG. 1. For this purpose, anassembly is provided comprising a multilayer EUV mirror (2) composed ofalternating Si and Mo layers deposited on a (e.g., silicon) substrate(1). A 2.5 nm Ru capping layer (3) is present on the multilayer EUVmirror (2). In order to provide a cavity over the EUV mirror (2), thefollowing steps can be performed at a temperature below 180° C. A SiO₂layer (4) is provided on the Ru capping layer (3) by ALD. A SoC layer(5) is deposited on the SiO₂ layer (4) by spin-coating. A SoG layer (6)is deposited on the SoC layer (5) by spin-coating. A photoresist (7) isprovided on the SoC layer (5) (e.g., on the SoG layer (6)). Thephotoresist (7) is patterned with an opening which laterally extendssuitably for providing the cavity in the SiO₂ layer (4) after transfertherein. In an alternative embodiment, the SoG (6) and SoC (5) layersmay be replaced by an SiOC and an amorphous carbon (APFtm) layerrespectively. The SoG (6) and SoC (5) layers can be preferred becausetheir use involves lower temperatures.

We now refer to FIG. 2. The pattern of the photoresist (7) istransferred into the SiO₂ layer (4) by a dry etching process. For thispurpose, SoG (6) is opened with a fluorine-based plasma such as CF₄.Then, SoC (5) is opened with a N₂/H₂ plasma. The photoresist (7) isconsumed during this step. Then, SiO₂ (4) is etched with aC₄F₈/CHF₃/CF₄/O₂ plasma. The SoG (6) is consumed during this step. Thisetching stops at the Ru capping layer (3) which serves as an etch stoplayer. The remaining SoC (5) is then stripped with an O₂ plasma or aN₂H₂ plasma. In an alternative embodiment, the Ru capping layer has a Colayer or a Ni layer thereon to serve as a seeding layer for theformation of the EUV absorbing structure. In that embodiment, theetching of the SiO₂ (4) stops at that Co layer or Ni layer.

We now refer to FIG. 3. A 33 nm metal layer (8) (e.g., Co) is grownselectively in the cavity formed in the patterned SiO₂ layer (4) by ELD.In alternative embodiments, a Ni layer, a material comprising cobalt(e.g. CoWP) or a material comprising Ni (e.g. NiPt or Ni doped with B)can be grown instead of pure Co.

We now refer to FIG. 4. The patterned SiO₂ layer is removed selectivelywith respect to the Ru capping layer and the deposited metal withdiluted HF.

With the obtained EUV absorbing structure being somewhat thin (33 nm),shadowing effects are low.

EXAMPLE 2 Formation of a Reticle Including a Patterning Step into an SoCCore

Example 2 is related to forming a reticle by using an organic layer (aspin-on-carbon layer) as a template for the area selective deposition ofCo or Ni or compounds comprising the same. We now refer to FIG. 5. Anassembly is provided comprising a multilayer EUV mirror (2) composed ofalternating Si and Mo layers deposited on a substrate (1). A 2.5 nm Rucapping layer (3) is present on the multilayer EUV mirror (2). In orderto provide a cavity over the EUV mirror, the following steps can beperformed at a temperature below 180° C. A SoC layer (5) is provided onthe Ru capping layer (3) by spin-coating. An SoG layer (6) is depositedon the SoC layer (5) by spin-coating. A photoresist (7) is provided onthe SoG layer (6). The photoresist (7) is patterned with an openingwhich laterally extends suitably for providing the cavity in the SoClayer (5) after transfer therein.

We now refer to FIG. 6. The pattern of the photoresist (7) istransferred into the SoG layer (6) by a dry etching process. SoG (6) isopened with a fluorine-based plasma such as CF₄. Then, SoC (5) is openedwith a N₂/H₂ plasma. The photoresist (7) is consumed during this laststep. This etching stops at the Ru capping layer (3) which serves as anetch stop layer. Remaining SoG (6) can be stripped with diluted HF whichselectively etches SoG (6) with respect to SoC (5) and Ru (3).

We now refer to FIG. 7. A 32 nm metal layer (8) (e.g., Ni) is grownselectively in the cavity formed in the patterned SoC layer (5) by ELD.In alternative embodiments, a Co layer, a material comprising cobalt(e.g. CoWP) or a material comprising Ni (e.g. NiPt or Ni doped with B)can be grown instead of pure Ni.

We now refer to FIG. 8. The patterned SoC layer (5) is removedselectively with respect to the Ru capping layer (3) and the depositedmetal layer (8) with a O₂ or N₂/H₂ plasma.

With the obtained EUV absorbing structure being somewhat thin (32 nm),shadowing effects are low. The use of a SoC (5) for the side walls ofthe cavity as compared to SiO₂ (4) for the same purpose may allowobtaining an angle closer to 90° between the side walls and the mirror.

EXAMPLE 3 Formation of a Reticle Including a Cavity Forming Step into anSoC Core with a SAM at the Bottom of the Cavity

Example 3 is related to forming a reticle by using an organic layer (5)(a spin-on-carbon layer) as a template for the area selective depositionof the metal layer (8) (e.g., Co, Ni, or compounds comprising the same)while introducing a SiO₂ or Si₃N₄ layer (10) as a protection layer onthe Ru capping layer (3). We now refer to FIG. 9. An assembly isprovided comprising a multilayer EUV mirror (2) composed of alternatingSi and Mo layers deposited on a Si substrate (1). A 2.5 nm Ru cappinglayer (3) is present on the multilayer EUV mirror (2). A SiO₂ or Si₃N₄layer (10) is deposited on the Ru capping layer (3) by a CVD process. Inorder to provide a cavity over the EUV mirror, the following steps canbe performed at a temperature below 180° C. A SoC layer (5) is providedon the SiO₂ or Si₃N₄ layer (10) by spin-coating. An SoG layer (6) isdeposited on the SoC layer (5) by spin-coating. A photoresist (7) isprovided on the SoG layer (6). The photoresist (7) is patterned with anopening which laterally extends suitably for providing the cavity in theSoC layer (5) and part of the SiO₂ or Si₃N₄ layer (10) after transfertherein.

We now refer to FIG. 10. The pattern of the photoresist (7) istransferred into the SoG layer (6) by a dry etching process. SoG (6) isopened with a fluorine-based plasma such as CF₄. Then, SoC (5) is openedwith a N₂/H₂ plasma. The photoresist (7) is consumed during this step.This etching is stopped at the SiO₂ or Si₃N₄ layer (10) which serves asan etch stop layer. Remaining SoG (6) can be stripped with diluted HFwhich selectively etches SoG (6) with respect to SoC (5) but etches partof the thickness of the SiO₂ or Si₃N₄ layer (10).

We now refer to FIG. 11. A SAM (11) is applied on the SiO₂ or Si₃N₄layer (10) now accessible in the cavity. This SAM (11) comprises areactive group for attaching selectively to the SiO₂ or Si₃N₄ layer(10). An example of a SAM is a silane group. This SAM (11) furthercomprises a group promoting metal deposition and therefore impartingselective metal deposition with respect to the other surfaces of thereticle under construction. An example of such groups are thiol groupsand polar groups. The presence of these thiol groups generally increasesthe selectivity of the metal layer (8) deposition. The SAM (11)generally has a hydrophobic chain linking, on one hand, the reactivegroup for attaching selectively to the SiO₂ or Si₃N₄ layer, and on theother hand, the group promoting metal deposition. The hydrophobic chainmay for instance be a hydrocarbyl comprising from 2 to 20 carbon atoms.The metal deposition can be performed by ELD via the use of a Pdcatalyst and a metal precursor.

We now refer to FIG. 12. A 33 nm metal layer (8) (e.g., Co) is grownselectively in the cavity formed in the patterned SoC layer (5) by ELD.In alternative embodiments, a Ni layer, a material comprising cobalt(e.g. CoWP) or a material comprising Ni (e.g. NiPt or Ni doped with B)can be grown instead of pure Co.

We now refer to FIG. 13. The patterned SoC layer (5) is removedselectively with respect to the SiO₂ or Si₃N₄ (10) and the depositedmetal layer (8) with an O₂ or N₂/H₂ plasma.

We now refer to FIG. 14. The SiO₂ or Si₃N₄ (10) can be etchedselectively with respect to the Ru (3) and the metal layer (8) (e.g.,Co) by using the metal layer (8) as a mask. If Si₃N₄ (10) is used, thisetching can for instance be performed by using a CHF₃/CF₄/O₂ plasma.

With the obtained EUV absorbing structure being thin (33 nm), shadowingeffects are low. The use of a SoC (5) for the side walls of the cavityas compared to SiO₂ (4) for the same purpose may allow obtaining anangle closer to 90° between the side walls and the mirror.

EXAMPLE 4 Formation of a Reticle Comprising an Embedded EUV AbsorbingStructure

Example 4 is related to forming a reticle by using the multilayer EUVmirror as a template for the area selective deposition of a metal layer(8) (e.g., Co, Ni, or compounds comprising the same). We now refer toFIG. 15. An assembly is provided comprising a multilayer EUV mirror (2)composed of alternating Si and Mo layers deposited on a Si substrate(1). A 2.5 nm Ru capping layer (3) is present on the multilayer EUVmirror (2). A SiO₂ or Si₃N₄ layer (10) is deposited on the Ru cappinglayer (3) by a CVD process. In order to provide a cavity in the EUVmirror (2), the following steps can be performed at a temperature below180° C. A SoC layer (5) is provided on the An SiO₂ layer (10) byspin-coating. An SoG layer (6) is deposited on the SoC layer (5) byspin-coating. A photoresist (7) is provided on the SoG layer (6). Thephotoresist (7) is patterned with an opening which laterally extendssuitably for providing the cavity in the EUV mirror (2) after transfertherein.

We now refer to FIG. 16. The pattern of the photoresist (7) istransferred into the SoG layer (6) by a dry etching process. SoG (6) isopened with a fluorine-based plasma such as CF₄. Then, SoC (5) is openedwith a N₂/H₂ plasma. The photoresist (7) is consumed during this step.This etching is stopped at the SiO₂ layer (10) which serves as an etchstop layer. The SiO₂ layer (10) is then opened with a CHF₃/CF₄/O₂/C₄F₈plasma. The SoG layer (6) is consumed during this step. The Ru cappinglayer (3) is etched with an O₂/Cl₂ plasma. The cavity is thentransferred into the EUV mirror (2) by partially etching through it witha Cl₂ reactive ion etch or with a SF₆/CH₂F₂/He/N₂ plasma.

We now refer to FIG. 17. A SAM layer (12) is selectively applied on theRu metal layer. A SAM layer (12) is for instance a SAM layer (12) havingof general formula R—SH having a thiol group for attaching to the Rulayer and a R group for preventing or inhibiting metal layer (8)deposition.

We now refer to FIG. 18. A 33 nm metal layer (8) (e.g., Co) is grownselectively in the cavity formed in the patterned EUV mirror (2) by ELD.In alternative embodiments, a Ni layer, a material comprising cobalt(e.g. CoWP), or a material comprising Ni (e.g. NiPt or Ni doped with B)can be grown instead of pure Co.

EXAMPLE 5 Formation of a Reticle Comprising a Multilayer EUV AbsorbingStructure

Example 5 is related to forming a reticle by using a SoC template forthe area selective deposition of a multilayer EUV absorbing structure.FIG. 19 depicts a structure remaining after performing steps depicted inFIGS. 5 and 6 of example 2.

We now refer to FIG. 19. A metal layer (8) (e.g., Ni) is grown by ELDselectively in the cavity formed in the patterned SoC layer (5). Aspacer layer (13) is then grown on the metal layer (8) by ALD. Eachlayer (8, 13) is thinner than 6 nm. In alternative embodiments, a Colayer, a material comprising cobalt (e.g. CoWP) or a material comprisingNi (e.g. NiPt or Ni doped with B) can be grown instead of pure Ni. Forthe spacer layer, TiN or MgO can, for instance, be used. Generally,after each deposition of a spacer layer (13), a treatment of the spacerlayer (13) surface is performed to allow the deposition of a furthermetal layer (8) (e.g., Ni) thereon.

We now refer to FIG. 20. Further metal layers (8) (e.g., Ni) and spacerlayers (13) are grown alternatively until, e.g., a thickness equal to3.5 times a bilayer of a metal layer (8) (e.g., Ni)/spacer layer (13) isobtained. This alternation of metal layer (8) and spacer layers (13) canreduce the tendency of the metal to crystalize. It also can allow phasetuning. The thickness of the bilayer may be adapted to improve the focusshift.

EXAMPLE 6 A Substrate without EUV Mirror

We now refer to FIG. 21. Example 6 is related to growing cobaltselectively in a cavity formed in SiO₂. A Si substrate (1) was provided.A 2.5 nm Ru capping layer (3) was present on the Si substrate (1). A 3nm Co layer was provided on the Ru layer (3) by ELD. In order to providea cavity over the EUV mirror, the following steps can be performed at atemperature below 180° C. A SiO₂ layer (4) was provided on the 3 nm Colayer by ALD. A SoC layer (5) was deposited on the SiO₂ layer (4) byspin-coating. A SoG layer (6) was deposited on the SoC layer (5) byspin-coating. A photoresist (7) was provided on the SoC layer (5). Thephotoresist (7) was patterned with openings which laterally extendsuitably for providing 310 nm wide trenches separated by 300 nm wideSiO₂ walls after transfer of the pattern in the SiO₂.

The pattern of the photoresist (7) was transferred into the SiO₂ layer(4) by a dry etching process. For this purpose, SoG (6) was opened witha CF₄ plasma. Then, SoC (5) was opened with a N₂/H₂ plasma. Thephotoresist (7) was consumed during this step. Then, SiO₂ (4) was etchedwith a C₄F₈/CHF₃/CF₄/O₂ plasma. The SoG (6) was consumed during thisstep. This etching stopped at the 3 nm cobalt layer which served as anetch stop layer. The remaining SoC (5) was then stripped with an O₂plasma or a N₂H₂ plasma.

The resulting openings in the SiO₂ were 310 nm wide and 37 nm deep.

A 29 nm metal layer (8) (e.g., Co) was grown selectively in the cavityformed in the patterned SiO₂ layer (4) by ELD.

We now refer to FIG. 22. The same example was repeated but thephotoresist (7) was patterned with openings which laterally extendsuitably for providing 110.5 nm wide trenches separated by 200 nm wideSiO₂ walls after transfer of the pattern into the SiO₂. The resultingopenings in the SiO₂ were 110.5 nm wide and 37.7 nm deep. These openingswere completely filled with Co.

We now refer to FIG. 23. The same example was repeated but thephotoresist (7) was patterned with openings which laterally extendsuitably for providing ˜100 nm wide trenches separated by ˜100 nm wideSiO₂ walls after transfer of the pattern into the SiO₂. The resultingopenings in the SiO₂ were ˜100 nm wide and ˜40 nm deep. These openingswere completely filled with Co.

Comparative Example on a Substrate without EUV Mirror

The purpose of this comparative example was to test if a reticle couldbe formed by directly etching the desired reticle EUV absorber patternby subtracting material from a Ni layer present on a Ru layer on asubstrate. For this purpose, an assembly was provided comprising a 2.5nm Ru capping layer present on a Si substrate. A 25 nm Ni layer was thendeposited on the Ru layer. A 5 nm Ru layer was deposited on the Ni layerto serve as an etch stop layer. A 100 nm TiN layer was deposited on theRu to serve as a hard mask. A 50 nm SiO₂ layer was deposited on the TiNlayer, a 130 nm SoC layer was deposited on the SiO₂ layer, a 28 nm SoGlayer was deposited on the SoC layer, and a photoresist was depositedand patterned on the SoG layer. This pattern was transferred to the hardmask in a series of etching steps. At this stage, two methods were triedto transfer the pattern into the Ni layer. The first method consisted inperforming a physical reactive ion etch on the 5 nm Ru and the 25 nm Nilayer. The second method consisted of performing an ion beam etch onthese same layers. In both cases, controllably stopping the etch at thebottom 2.5 nm Ru layer was not achieved. Indeed, both these methodssuffered from a lack of selectivity toward Ni with respect to Ru.Furthermore, both methods lead to re-deposition of sputter Ni residue onthe 2.5 nm Ru, away from where Ni was desired. Also, the profileobtained for the Ni structure was difficult to control for both methods.

It is to be understood that although embodiments, specific constructionsand configurations, as well as materials, have been discussed herein fordevices according to the present disclosure, various changes ormodifications in form and detail may be made without departing from thescope of this disclosure. For example, any formulas given above aremerely representative of procedures that may be used. Steps may be addedor deleted to methods described within the scope of the presentdisclosure.

While some embodiments have been illustrated and described in detail inthe appended drawings and the foregoing description, such illustrationand description are to be considered illustrative and not restrictive.Other variations to the disclosed embodiments can be understood andeffected in practicing the claims, from a study of the drawings, thedisclosure, and the appended claims. The mere fact that certain measuresor features are recited in mutually different dependent claims does notindicate that a combination of these measures or features cannot beused. Any reference signs in the claims should not be construed aslimiting the scope.

What is claimed is:
 1. A method for making a reticle, comprising: providing an assembly comprising: an extreme ultraviolet mirror; a cavity overlaying at least a bottom part of the extreme ultraviolet mirror; a first self-assembled monolayer of a first type present in the cavity; and a second self-assembled monolayer of a second type present on a surface not forming part of the cavity; and at least partly filling the cavity with an extreme ultraviolet absorbing structure comprising a metallic material comprising an element selected from Ni, Co, Sb, Ag, In, and Sn, by forming the extreme ultraviolet absorbing structure selectively in the cavity, wherein the first self-assembled monolayer is configured to promote selective formation of the extreme ultraviolet absorbing structure in the cavity, and the second self-assembled monolayer is configured to resist formation of the extreme ultraviolet absorbing structure thereon.
 2. The method according to claim 1, wherein the cavity is either: comprised in a dielectric mask layer overlaying the extreme ultraviolet mirror, the cavity having a depth extending from a top surface of the dielectric mask layer to a bottom surface of the dielectric mask layer, or comprised in the extreme ultraviolet mirror, the cavity having a depth extending from a top surface of the extreme ultraviolet mirror to a level above a bottom surface of the extreme ultraviolet mirror.
 3. The method according to claim 1, wherein the extreme ultraviolet absorbing structure comprises layers of metallic extreme ultraviolet absorbing material alternated with spacer layers.
 4. The method according to claim 1, wherein forming the extreme ultraviolet absorbing structure selectively in the cavity comprises forming the extreme ultraviolet absorbing structure by electroless deposition.
 5. The method according to claim 1 wherein the assembly further comprises a capping layer on the extreme ultraviolet mirror and wherein the cavity exposes part of the capping layer.
 6. The method according to claim 5, wherein the capping layer comprises Ru.
 7. The method according to claim 1 wherein the assembly further comprises a capping layer on the extreme ultraviolet mirror and an etch stop layer on the capping layer, wherein the cavity exposes part of the etch stop layer.
 8. The method according to claim 1, wherein the extreme ultraviolet absorbing structure is embedded in the extreme ultraviolet mirror so that a bottom surface of the extreme ultraviolet absorbing structure is lower than a top surface of the extreme ultraviolet mirror and that a top surface of the extreme ultraviolet absorbing structure is higher or at the same level as the top surface of the extreme ultraviolet mirror.
 9. The method according to claim 1, wherein a seed layer overlays the extreme ultraviolet mirror and wherein the cavity opens on the seed layer.
 10. The method according to claim 1, wherein the cavity is formed by providing a dielectric mask layer over the extreme ultraviolet mirror followed by patterning the dielectric mask layer so as to form the cavity.
 11. The method according to claim 10, further comprising removing the dielectric mask layer selectively with respect to: the extreme ultraviolet absorbing structure, and the extreme ultraviolet mirror or a capping layer.
 12. The method according to claim 1, wherein a thickness of the extreme ultraviolet absorbing structure is 60 nm or less.
 13. The method according to claim 1, wherein a thickness of the extreme ultraviolet absorbing structure is 50 nm or less.
 14. The method according to claim 1, wherein a thickness of the extreme ultraviolet absorbing structure is 35 nm or less.
 15. The method according to claim 1, wherein the extreme ultraviolet absorbing structure comprises a layer consisting of Ni, Co, Ag, CoWP, or Ni doped with up to 20% B or NiPt by atomic composition.
 16. The method according to claim 1, wherein a thickness of the extreme ultraviolet absorbing structure is 50 nm or less.
 17. The method according to claim 1, wherein a thickness of the extreme ultraviolet absorbing structure is 35 nm or less. 